Power transistors are found in many common electronic devices, including power supplies, voltage converters, integrated circuits, and low-voltage motor controllers. Laterally diffused metal oxide semiconductor (LDMOS) transistors are one type of power transistor and are also used in microwave and radio frequency (RF) power amplifiers, for example. These transistors are often fabricated on p/p+ silicon epitaxial layers.
LDMOS transistors are MOS transistors that also have a drain drift region. The drain drift region, which touches and lies between the drain and the channel region, has the same conductivity type as the drain, but a lower dopant concentration than the drain. A depletion region forms in this lightly doped lateral diffused region resulting in a voltage drop between the drain contact and the transistor gate. With proper design, sufficient voltage may be dropped between the drain contact and the gate dielectric to allow a low gate voltage transistor to be used as a switch for the high voltage.
A large power transistor may be made up of many “fingers,” each of which can be long and may be considered to include multiple sub-transistors.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.